The hardening of ink and coating films can take place by evaporation of solvents, by absorption of solvents in the substrate, by oxidation, by polymerization in UV or electron beam dryers, or by a combination of the above-mentioned drying mechanisms.
Many factors argue for monitoring the hardening of ink and coating films. A sufficient drying or hardening of the printed inks/coatings is a prerequisite for high-quality printing. Inside the conveyance path of sheets through a machine, contact of the freshly printed or coated sheet surface can lead to damage, which results in print soilage. The sheets in the delivery unit of a sheet-fed printing machine, or the rolled up webs of a web-fed printing machine must be sufficiently dry with respect to the printed inks/coatings, since otherwise they would become smudged and stuck together during stacking or winding. The latter in particular would hinder further processing of the printed products or render it impossible.
Strict requirements must be placed on the adjustment of differently constructed dryer units. When the printing machine is being set up, the user must make adjustments to the dryer units in order to be able to produce the necessary quality within as short a time as possible and with as small a number of wasted sheets (feeding waste) as possible. An excessively high setting of the dryer power not only causes high power consumption of the printing machine and likewise a large waste of heat power, which would raise the temperature of the printing room or have to be adjusted to the predetermined value by the air-conditioning unit of the printing room, but also negatively affects the printing process itself. It is known that the tack and viscosity of ink/coating vary, so that under certain circumstances a high heating of the machine as well as the heating of the ink and the metering device could require re-regulation, causing waste. Excessively low settings for the dryer action or dryer power must likewise be avoided, since otherwise the above-described danger of smudging of freshly printed sheets during sheet transport or in the delivery device exists.
By controlling the dryer, it is possible not only to reduce print spoilage, but also to save considerable energy. Since the dryer devices are only operated at the power that is necessary according to the current situation, the power consumption of the devices is reduced to the respective required value. Reduced heating of machine elements or the entire machine results, because the dryer devices also reduce waste heat. The service life of the machine or of machine components is thereby increased. Finally, a reduced dryer power also avoids raising the printing room temperature. For an air-conditioned room this likewise provides energy savings since less waste heat is produced by the dryer.
Special issues arise for the printing of radiation-hardening inks and coatings in printing machines, which require monitoring of the hardening. Printing with radiation-hardening offset ink has become widely disseminated and is well known. The advantages of printing with radiation-hardening inks lie in the rapid spontaneous polymerization after irradiation with a UV radiation source, the lack of solvents in the ink, and the good printability of non-absorptive substrates.
Conventional inks, on the other hand, are either oil-based or contain solvents. Oil-based inks dry by oxidation of the oil-based binders, or they dry by evaporation of the easily volatilized oils in the ink (heat set method). In contrast, radiation-hardening inks and coatings are hardened by a photochemical process, also referred to as polymerization. The liquid or unpolymerized ink film is transformed into a solid state by polymerization under the influence of UV light.
Malfunctions are known, however, in which unpolymerized ink components are transferred to the back of the substrate above them by blotting in the delivery stack of a sheet-fed printing machine or by blotting in a reel after printing. Ink components can also migrate through the substrate. For packaging, migration of ink components or blotting of ink can lead to a sensory effect on the packaged material. If specific migration limits are exceeded in food packaging, harm to the health of consumers cannot be ruled out. If the specific migration limits are exceeded, packages must be withdrawn from the market, which, in addition to the financial consequences, results in lost reputation of the brand's manufacturer. Maintenance of harmlessness to health is paramount.
According to §30 LMBG [Food Safety Act], it is prohibited to manufacturer consumer articles in such a manner that, when used as directed, they are liable to be harmful to health due to their material composition, in particular, due to toxicologically active substances or contamination. Moreover, according to German and European law, and in the USA as well, the “no migration principle” applies, i.e., transfer of substances to the packaged foodstuffs must be avoided. Therefore, it is particularly important to ensure the polymerization of the radiation-hardening ink on the substrate in all cases.
Incompletely hardened radiation-hardening inks and coatings also have effects on the safety of the operating personnel. For example, if such ink or coating films on the substrate are not completely hardened and a machine operator takes samples to check the print quality, migration-capable components of the ink may be absorbed through the skin. In addition to health risks, irritation and allergic skin reactions can occur. The printing plant also incurs additional disposal costs from incompletely hardened coatings and inks, since printed sheets with unpolymerized ink or coating components must generally be treated as hazardous waste. This requires extra logistical effort and generates excess costs for disposal.
Many factors therefore suggest that the hardening or drying of ink and coating films on the substrate should be measured, measurement parameters formed, and they should be used to control the dryer directly or via user input. Additionally, a quality protocol can be formed from these measured values, which will give the end-user verification that the ink or coating has been sufficiently hardened. This is often demanded specifically in the field of radiation-hardening inks and coatings.
The desire to evaluate the hardening or drying of ink and coating films has already been widely discussed. In EP 1 142 711 B1 it was proposed among other things that a controller be provided for the dryer device, to which at least one signal of a parameter characterizing the printing process is supplied, and that, consequently, the controller generate signals for varying the operating mode of the dryer device in a predetermined manner. It is also proposed to undertake the controlling of the dryer unit on the basis of values measured on the substrate. The coating or ink film thickness and/or the gloss of the applied coating are mentioned as measurement parameters.
DE 19 737 785 A1 proposes to determine the degree of dryness of a coating film by measuring the intensity of a microwave signal that interacts with the coated substrate. The state parameter formed from the measurement signal can then be utilized as a control parameter for controlling the dryer. However, the method is suitable only for use of coating media that absorb microwave energy to a large extent, such as dispersion varnish, which has essentially only water as a solvent. Testing the polymerization of hardened ink and coating films was not mentioned in the patent.
DE 24 58 935 A1 discloses a method for measuring and controlling the speed of printing and coating machines, in which the wavelengths corresponding to the solvents that are used are continuously analyzed on the paper webs, with the average values measured after drying being compared to the preset maximum values. The measurement unit contains an analyzer head with continuous emission of radiation and a receiver for receiving the radiation that the product to be analyzed reflects or that passes through the latter. The disadvantage of this method is that the solvent composition must be known in advance. This is relatively simple for printing processes that operate with simple solvent compositions, but the complexity increases for modern ink with a number of solvents. The supplier generally discloses the composition of the ink only in approximate amounts of the constituents. This method also fails for printing processes that do not emit any solvent. These include, among others, radiation-hardening printing processes, as well as oxidative and ink-absorption processes. This measurement method is not universally applicable. Moreover, using it for fast-running printing machines is not known, since the analysis is time-intensive and complex.
Optical spectroscopy is used in the laboratory for assessing polymerization. Due to its complexity and the required preparation of specimens, this method is not well-suited for use in the rough machine environment of a printing plant. Ultrasound measurements are known as a measuring method in the printing industry. Test methods with ultrasound have the advantage that they are noninvasive and nondestructive techniques for determining the properties of a material.
One known approach for checking the strength of paper webs is the measurement of the propagation velocity and intensity of acoustic waves in these materials. Such a method and a device for nondestructive examination of paper in a continuous, rapidly moving web during manufacturing are described in DE 3 045 581 A1. This method exploits the fact that many strength parameters of paper are related to a modulus of elasticity. The latter can be correlated with the velocity of acoustic waves propagating through the paper web.
A transmitting acoustic source or transducer transmits a mechanical signal to the paper, and a receiver receives the ultrasound signal from the paper. With knowledge of the time that ultrasonic signals require for propagation through the paper and of the distance that these acoustic waves travel, the velocity of the acoustic waves can be calculated. The transducers are arranged according to this invention on wheels that are in physical contact with the paper web. Because of this contact and the strict requirements for synchronization of the wheels, the invention is not very practical. Such a measurement on printed paper webs appears to be very difficult due to the contact.
A measurement in which there is a non-contacting coupling of acoustic waves would therefore be desirable. As a non-contacting measurement of the flexibility of banknotes, DE 103 18 104 A1 proposes irradiating them with acoustic waves, detecting the acoustic waves coming from the sheet material, measuring the transmitted and reflected acoustic waves, and deriving values for the flexibility of the sheet material therefrom that are independent of the degree of soiling of the banknote. The disadvantage of the solution that was found is, among other things, that the acoustic receivers are arranged on both sides of the paper. Moreover, only transmitted or reflected acoustic waves are evaluated; the propagation of the acoustic waves on the surface is not taken into account.
Assessment with SAW (surface acoustic waves) is also known in science as a method for nondestructive testing processes.